During the processing of electrical signals for signal transformation, analysis or correction, frequently the timing of the signal must be altered or compensated. For example, signal timing compensation is commonly employed to correct undesirable timing differences between signals having recurrent timing components. Alteration of signal timing to correct undesirable timing differences is particularly important when a signal undergoes transformations between different domains, such as occur in recording and reproducing signals on magnetic or other forms of record media. During the recording and reproduction processes, the time function of the signal is transformed into a space function and then back into the time function. As the signal undergoes the transformations, timing or time-base errors are often introduced to the signal. The dynamic or time variant class of such time-base errors prevents the achievement of the necessary transient-free and time-stable signal reproduction required in high resolution signal processing systems. For example, time-stable signal generation is desirable in all television signal processing systems and highly stable generation mandatory in systems used to prepare television signals for public transmission.
Two techniques are employed to correct undesirable timing errors in signals reproduced from a record medium; electro-mechanical and electronic. Electro-mechanical techniques are employed to correct gross timing errors and achieve such correction by synchronizing the operation of the signal recording and reproducing equipment. Electronic techniques are employed to correct smaller residual timing errors not corrected by the electro-mechanical devices and achieve such correction by phase adjusting the signal. It is the electronic technique of time-base error correction to which the present invention is relevant.
Heretofore, electronic signal timing alteration systems have employed adjustable time delay devices inserted in the signal path to correct time-base errors. In such systems, the time-base error is measured and the amount of time delay inserted in the signal path adjusted to compensate for and, thereby, correct the measured time-base error. One particular type of system which enjoys widespread use has a voltage variable delay line in which lumped constant inductors and voltage variable capacitive diodes are interconnected in a delay line configuration. A voltage, corresponding to the measured time-base error, is applied to the variable capacitive diodes to fix the necessary delay for correcting the time-base error. A description of a voltage variable delay line type signal timing alteration system can be had by reference to U.S. Pat. No. 3,202,769.
In another type of electronic signal timing alteration system, a number of fixed delay lines or a single delay line with a series of taps spaced therealong are arranged in combination by electronic switches. Time-base errors are corrected by operating the switches in accordance with the measured error to selectively insert the necessary corrective delay in the signal path. A fixed delay line type signal alteration system is described in U.S. Pat. No. 3,763,317 and a tapped delay line type signal timing alteration system is described in U.S. Pat. No. 3,748,386.
Recently, digital delay devices, such as clocked storage registers, have been used in systems for correcting time-base errors in analog signals. In the digital systems, the analog signal being corrected is digitized, corrected and regenerated. Correction is performed by entering or writing the digitized signal in an adjustable storage register at a fixed rate determined by a reference clock signal. The storage register is adjusted to correct timing errors by reading the signal from the register at a faster or slower rate, depending upon the error. This technique of constant write rate and variable read rate cannot handle large discontinuous or incremental time-base changes in the signal. In tape recorders, such incremental time-base changes are caused by anomalies in their operation and most commonly when switching between heads.
In signal timing alteration systems, especially those arranged to eliminate time-base errors and provide a high degree of signal time-base stability, it has been the practice to cascade coarse correction devices and fine correction devices. Voltage variable delay line systems have been used to provide the desired fine correction while switched delay line systems have been used to provide the coarser corrections. Because all such delay line systems are analog devices, they are prone to drift and have other disadvantages characteristic of analog devices. Incremental time-base changes that occur as a result of anomalies in the operation of tape recorders often cause errors or costly interruptions in the performance of signal processing operations because of the inability of these time-base error correction devices to respond to the incremental changes. Also, if a large range of timing errors is required to be corrected, large and complex correction systems are necessary.
Considerable advantage is therefore to be gained by utilizing a technique to perform signal timing compensation that is able to effect all time alterations, including incremental, without error. Additional advantages will be realized in the performance of such signal timing compensation by first altering the signal timing by any fraction of a known interval required to bring the signal within an integral number of known intervals of the desired timing and, thereafter, altering the signal timing by such integral number of known intervals.